The present invention relates to an improved clad material, and more particularly relates to improvement in construction of a Ni-Ti type clad material which includes core and sheath components.
A Ni-Ti type clad material is usually made up of a Ti base metallic core and a Ni base metallic covering or sheath clad to each other. Ti base metal usually has light weight, excellent mechanical strength and high workability whereas Ni base metal has excellent suitability for brazing and plating with beautiful surface luster. Thanks to combination of such advantages of the component metals, Ni-Ti type clad materials are expected to be well used for production of eyeglass-frames, watch-bands and other decorative accessories. In practice, however, use of Ni-Ti type clad material for these purposes is enevitably accompanied with several problems.
First, for the above-described usages, the material has to be subjected, after annealing, to plastic works such as rolling and drawing, mechanical works such as griding, brazing and plating. In connection with these works, however, Ni-Cr alloys used for the Ni base metallic sheath is usually poor in plastic workability. In order to enable cladding with the Ti base metallic core and subsequent plastic works, the Ni-Cr alloys have to be subjected to annealing at temperatures of 600.degree. C. or higher. Heating at such high temperatures generates inter-metallic compounds such as Ti.sub.2 Ni, TiNi and TiNi.sub.3 at the boundary between the core and the sheath, which cause separation between the two components and lowering in brazing strength.
Next, sole use of Ni for the sheath enables annealing at relatively low temperatures, thereby successfully avoiding the above-described problem of inter-metallic compound. However, the too soft and ductile nature of Ni results in predominant stretch of the Ni sheath, when compared with behavior of the Ti core, during the plastic deformation, in particular at drawing, thereby causing disorder in clad ratio and/or seizure to drawing dies. Further, in rolling for square rods, the easy stretch of the Ni sheath disenables correct production of square rods as designed, or to say the least, neccessitates difficult and complicated control of the rolling conditions.